This invention relates to a predictive encoder for use in a predictive code communication system.
A predictive code communication system is suitable for the efficient transmission of highly redundant signals such as a television video signal which has high correlations. Such system is generally classified into three systems: an intraframe code communication system, an interframe code communication system, and a combined predictive code communication system. In the intraframe and interframe code communication systems, the preceding sample feedback is used to produce predictive error signals. The correlation of the television signal permits the preceding samples as predicted samples. More specifically, the predictive error signal is provided by feeding back the samples of a preceding frame for comparison and subtraction. In the combined predictive code communication system, use is made of the interframe prediction and of intraframe prediction, the correlation in the latter being taken within a frame between, for example, two successive scanning lines.
In these systems, the predictive error signal has an amplitude distribution concentrated in a small amplitude region so that the information codes for smaller quantization levels appear more frequently to result in another kind of redundancy. To avoid such redundancy, variable length codes may be used assigning a code of shorter length to frequently occurring quantization levels.
The use of variable length codes makes it possible to decrease the amount of information to be transmitted by one bit per sample on the average as compared with the ordinary transmission by equal length codes. In practice, the decrease varies from zero to about two bits depending on the nature of the signal. A buffer memory is therefore used to temporarily store the variable length codes before transmitting them at a predetermined transmission bit rate. Overflow of the buffer memory is nevertheless inevitable for a signal portion carrying much information, namely, for rapidly varying information, when use is made of a transmission rate suitable to an average amount of the information.
In order to avoid the overflow, proposals have been made to change the quantization characteristics, e.g., to make it coarse or to reduce the number of the quantization steps depending on the amount of information. The changes in the quantization characteristics, however, deteriorate the quantization noise or strengthen the overload distortion to degrade the transmitted picture at the edge portion.
With the combined predictive encoding system, the highly effective interframe prediction can be expected with respect to a still-motion television signal and the highly effective intraframe prediction can be expected with respect to a television signal for a rapidly moving picture. In this system, however, a great amount of feedback is necessary to eliminate random noise and quantizing noise through a correlation operation. If the amount of feedback is not sufficient, the increase in the undesired redundancy components cannot be avoided in information to be transmitted.
In order to reduce such redundancy components, proposals have been made to insert an attenuating circuit having a gain less than 1 or a low pass filter in the interframe predictive loop. The insertion of such circuit elements, however, causes deterioration in picture such as blur at moving portions.